The present invention relates to a method for overlay welding of stainless steel on the inner surface of pressure vessels which are used in treating hydrogen at elevated temperatures and under high pressure.
The inner surfaces of the pressure vessels used for desulphurization and hydrocracking processes in oil refineries are usually overlaid with a stainless steel having a thickness of 4.5 to 8 mm for the prevention of corrosion. In the processes atomic hydrogen is absorbed from the surface of the stainless steel weld overlay and diffused into the base metal, and also it is diffused partially through out the wall of the base metal at elevated temperature and under high pressure. In this case, when the temperature of the vessel is lowered on shutdown of the plant, the hydrogen absorbed within the vessel wall makes the boundary layer between the stainless steel and the base metal brittle so that sometimes a cracking known as disbonding similar to a delayed failure may occur. Thus, the hydrogen in the boundary layer accumulates in the inside of the cracks which promotes the occurrence of blisters or surface cracks in the weld overlay. Therefore, it causes serious problems for the safety of the plant.
However, hitherto no systematic study has been carried out on the phenomenon of disbonding, so its causes and mechanism were not clear, but recent studies have pointed out that the principal origin of the disbonding resides in the difference of hydrogen solubility between the weld overlay of austenitic stainless steel and the ferritic base metal, or in the difference of heat expansion, and the microstructure in the boundary layer having a strong susceptibility to hydrogen embrittlement caused in a dissimilar weldment.
That is, according to the studies, since hydrogen absorbed in the wall of a vessel during operation makes the boundary layer between the base metal and the stainless steel weld overlay brittle, so it was considered that the cracks would be initiated and propagated due to increasing hydrogen gas pressure in the void and residual stress in the welded zone.
As the result of the studies by the inventors on methods of overlay welding which can prevent the disbonding a method was found which could attain this purpose by either a selection of a welding material, or the welding procedure of the first overlay cladding layer to be welded on the base metal has been improved.